The determination of dissolved oxygen in iron and steel by immersing an electrolytic cell and temperature sensing device into the molten metal during the refining of steel has been practiced by steel companies for many years. Because of the extremely harsh conditions under which these oxygen sensors are used, the solution of many problems associated with their use must be based upon assumptions as to the cause of the problem as well as the mechanism involved in their solution.
While there has been much improvement in the reliability of the readings from the oxygen sensors of this type over the years, there continues to be the problem of initial overshoot of the output signal upon immersion, i.e., the electromotive force initially moves in a positive direction beyond the true value and then decreases slowly stabilizing at the true value. One cause of this problem, as it deals with measurements in iron or steel having low levels of oxygen in the 2 to 3 parts per million range, was solved by the structure disclosed in U.S. Pat. No. 4,717,463, issued on Jan. 5, 1988, which is hereby incorporated by reference as a part of this application. That patent discloses, among other things, the use of an open ended steel tube 36 surrounding the solid electrolyte tube 24 and extending beyond the tube 24 as a means to reduce the thermal shock to the tube 24 upon immersion of the sensor into molten metal. In the referenced patent, the tube 36 is of such an inner diameter that it does not contact the tube 24, but provides an air space between the tube 24 and tube 36. Also, as stated in the referenced patent, the tube 36 is made of corrosion resistant steel to eliminated any possibility of rusting of the tube 36, which might adversely affect measurements of oxygen at low concentration.
In aluminum killed steel melts, which typically range from 0.03% to 0.05% Al with the range of dissolved oxygen in the 2 to 5 ppm range, it has become routine in recent practice to make a measurement of dissolved aluminum by using a dissolved oxygen sensor of the type disclosed in the referenced patent. The basis for using an oxygen sensor for this measurement is set forth below:
The reaction of dissolved aluminum and oxygen is given by EQU 2Al+3O=Al.sub.2 O.sub.3,
The equilibrium constant is ##EQU1## where EQU a.sub.Al.sbsb.2.sub.O.sbsb.3
is the activity of Al.sub.2 O.sub.3 with pure Al.sub.2 O.sub.3 as the standard state and EQU h.sub.Al and h.sub.o
are the activities of Al and O for the 1 wt. % standard state. Assuming EQU a.sub.Al.sbsb.2.sub.O.sbsb.3.sub.(s) =1,
then ##EQU2## and at constant temperature EQU log h.sub.Al =-0.5 log K-1.5 log h.sub.O.
For low alloy steel h.sub.O and h.sub.Al are equal to the weight percent of oxygen and aluminum, respectively. Because of this relationship, it can be seen that soluble aluminum (Al) is predictable as a function of dissolved oxygen (O).
In measuring the dissolved oxygen (oxygen activity) in aluminum killed (deoxidized), high silicon steel melts in which the dissolved aluminum is in the 0.03% to 0.07% range and % Si&gt;0.1%, the output of the sensors have frequently been found to be unstable in that they produce an initial overshoot of the true oxygen level before the sensor millivolt output stabilizes. The desired response, of course, is one which rises quickly to a voltage output related to the true oxygen level with substantially no overshoot. Such a desired response is usually obtained when % Si&lt;0.01%. A series of hypothesis were investigated to determine the cause of the undesirable overshoot. The most credible of these hypotheses is believed to be the one which postulates that there is a precipitation of silica (SiO.sub.2) on a "cold" sensor. The essence of this hypothesis is set forth below.
When the sensor first enters the liquid steel it is cooler than the steel and the following reaction occurs: EQU Si+2O.fwdarw.SiO.sub.2 (s).
As the sensor is heated by the molten steel, the reverse reaction proceeds until equilibrium is attained. Thus EQU SiO.sub.2 (s).fwdarw.Si+2O.
This affects the local oxygen activity, resulting in an unstable emf output, less negative than for the true oxygen activity. Thus EQU SiO.sub.2 =Si+2O.
A typical equilibrium constant for the above reaction is EQU log K=-31,000/T+12.0,
where EQU a.sub.SiO.sbsb.2 is the activity of SiO.sub.2
and EQU h.sub.Si is the activity of Si
for the 1 wt. % standard state, then ##EQU3## and for h.sub.Si .apprxeq.% Si and h.sub.O .apprxeq.% O, ##EQU4## Thus, if the activity of SiO.sub.2 is unity, precipitation of silica will occur when the product EQU [% Si][% O].sup.2 exceeds Ka.sub.SiO.sbsb.2.
For an Si content of 0.3%, pure SiO.sub.2, and a temperature of 1500.degree. C. (1773.degree. K.), this occurs when EQU O.sup.2 (0.3)&gt;10.sup.-31000/T+12.0 EQU O.sup.2 (0.3)&gt;3.28.times.10.sup.-6 EQU O&gt;33 ppm.
Thus, the precipitation of silica will only occur at oxygen levels greater than 33 ppm for a 0.3% Si, if the activity of silica is unity. However, the inclusions present in the steel are complex Al.sub.2 O.sub.3 -MnO-SiO.sub.2 inclusions and should, therefore, have a silica activity much less than one (1). For example, if the activity of silica is 0.01, the precipitation of silica at 1500.degree. C. and 0.3% Si can only occur above 3.3 ppm. If % SiO.sub.2 (in the inclusions).apprxeq.20%, the activity of silica is 0.005 (see "Activity of Silica in Calcium-Aluminate Based Slags", by Bahri Ozturk and R. J. Fruehan, Metallurgical Transactions B, pages 746-749, volume 18B, Dec. 1987). If the temperature is increased to 1550.degree. C., silica precipitation can only occur above 5.7 ppm. Thus, it appears that one should seek a sensor design that assures the oxygen cell is as close as possible to the molten steel temperature before molten steel contacts the cell. One critical constraint, of course, is that the total residence time of the sensor in the molten steel is limited to approximately 10 seconds because of operating conditions. Therefore, the delay of contact between the sensor and the steel bath cannot be too long or a sufficient response will not be obtained by the sensor.
The prior art related to the present invention is illustrated in FIGS. 1 and 2. FIG. 1 shows the relevant parts of the referenced U.S. Pat. No. 4,717,463. In the figures, the reference characters correspond to those in the referenced patent. Thus, the solid electrolyte tube 24 is in the form of a closed end tube of magnesium stabilized zirconia and it contains an oxygen reference material 26 above an inert filler 27 which maintains the material 26 in contact with the inner surface of tube 24. One electrical connection for the sensor is that made to the reference material 26 by conductor 28. The other electrical connection for the sensor is not shown in FIG. 1, but is shown in the referenced patent as being peripheral to the filler 34, which secures the parts of the sensor in the sensor body.
As will be noted, the prior art arrangement of FIG. 1 has an open end tube 36 which acts as a heat shield to reduce the thermal gradient along the solid electrolyte to minimize thermal breakage of that part. It is desirable to improve that arrangement to overcome the instabilities previously mention by increasing the delay between the time of immersion of the sensor and the time when molten metal contacts the solid electrolyte so that the solid electrolyte has a chance to heat up to a temperature closer to that of the bath itself than is possible with the arrangement of FIG. 1.
Another prior art arrangement is shown in FIG. 2, which illustrates the teaching of U.S. Pat. No. 4,342,633 as it would apply to the structures in U.S. Pat. No. 4,717,463. This arrangement shows the heat shield 36' as a closed end tube, suggested in the patent to be of low carbon steel, which is in contact with the solid electrolyte and serves to dampen thermal shock produced during immersion. With such an arrangement the low carbon steel of the shield would tend to cause unstable readings for the oxygen sensor because of the oxidation or rusting of the shield combined with the fact of contact between the shield and the solid electrolyte. As the shield melts, the oxygen associated with the iron oxide (rust) causes the oxygen sensor to indicate a higher oxygen activity than that which would be truly representative of the steel being tested.
It is an object of the present invention to provide an improved immersion type oxygen sensor capable of producing stable readings when the bath to be measured is an aluminum killed, high silicon melt.
According to the present invention improved sensor response is achieved with a shield protecting the solid electrolyte of the cell from contact with the steel bath until the temperature of the solid electrolyte has had a chance to more nearly approach the temperature of the bath than has been possible with the prior art with this protection being provided without contact between any oxidizing elements of the protecting shield and the solid electrolyte. Avoidance of such contact will avoid false readings due to oxidation of the shield itself.